The present invention relates to an image forming apparatus such as a digital copy machines which reads an image on an original sheet, processes the read image by an image processing devices and forms the image on a sheet.
Recently, image forming apparatus such as digital copy machines and facsimile devices have been rapidly popularized. In these digital office apparatus, the most important operation is the image processing for creating a high-quality image output from an image input by a scanner. Generally, the image input/output apparatus such as digital copy machines and facsimile devices, entail the problems of various types of deteriorations of space frequency as follows: the deterioration of the MTF (modulation transfer function) characteristics of the optical system for reading, and the generation of folded distortion caused by the limitation of the frequency band for digital sampling, which take place during the data inputting; the deterioration in the developing system; the generation of moire which is unique to digital process, and the like.
In order to avoid these problems, the image processing device requires an MTF correction. Such an MTF correction is called "filtering process", which is further divided into a low pass filtering (to be abbreviated as LPF) for suppressing moire and the like, and a high pass filtering (to be abbreviated as HPF) for emphasizing an edge or the like, of a character.
Conventionally, the LPF and HPF are carried out one at a time in series in the course of the flow of the entire image process, that is, an input image is subjected first to the LPF, then to the range correction process for correcting the contrast of the image, and to the HPF.
In general, the filtering process of a digital copying machine, facsimile or the like, is performed through a two-dimensional digital filtering process in the main scanning and the sub-scanning directions. More specifically, the filtering process is performed by multiplying each pixel of a local region consisting of a pixel to be processed (focused pixel) and peripheral pixels, by a coefficient which corresponds to a respective pixel.
Therefore, in the case where a process is performed by a mask size of (n.times.n), the process in the sub-scanning direction requires line buffers of n lines. However, the filtering process is carried out by the conventional method, the process is performed in a serial manner, and therefore in the case of the LPF and HPF process of a mask size of (3.times.3), line buffers of a total of 4 lines, two lines for each, are required.
As described above, with the conventional technique, in the case where a filtering process of an image processing apparatus is carried out by a mask size of (n.times.n), the process in the sub-scanning direction requires line buffers of n lines. With this structure, if the filtering process is carried out by the conventional method, the process is performed in a serial manner, and therefore in the case of the LPF and HPF process of a mask size of, for example, (3.times.3), line buffers of a total of 4 lines, two lines for each, are required.